This invention relates to an outboard motor exhaust system and more particularly to an improved exhaust system for an outboard motor having a four-cycle internal combustion engine.
As is well known, the compact nature of outboard motors creates a number of unique and difficult design challenges for engineers. One of the major areas where these design concerns arise is in connection with the exhaust system for the outboard motor. Because of the relatively small size available, the types of silencing arrangements utilized in other engine applications are not available.
Therefore, the silencing of the exhaust gases must be done in major part either in the drive shaft housing through which the exhaust gases exit to the atmosphere and/or by discharging the exhaust gases from an area below the water, thus utilizing the body of water in which the watercraft operates as a silencing medium.
The underwater exhaust gas discharge is useful in providing some silencing, but under many running conditions of the watercraft, the underwater discharge is too low to permit effective exhaust gas discharge. Thus, most outboard motor exhaust systems employ a high speed, underwater exhaust gas discharge and a more restricted, above the water, idle exhaust gas discharge.
This latter exhaust gas discharge is utilized to discharge the exhaust gases to the atmosphere through the drive shaft housing and above the body of water in which the watercraft is operating under idling, trolling, and other low engine speed operating condition.
Obviously, some silencing arrangement must be employed for silencing the exhaust gases that flow through the above the water, idle exhaust gas discharge. In addition, a silencing arrangement is also employed to assist in the underwater discharge for silencing of the exhaust gases when traveling at higher speeds.
The problems of effectively silencing the exhaust gases and discharging them to the atmosphere are significantly magnified when the outboard motor is powered by a four cycle internal combustion engine. Four cycle engines obviously require a lubricating system that has a fairly large reservoir for lubricant recirculation.
Normally, the lubricant is stored in a lubricant tank that is supported form the underside of the exhaust guide which is, in turn, positioned at the upper end of the drive shaft housing. The engine is mounted on this exhaust guide and the exhaust gases are discharged downwardly from the engine through the exhaust guide. This arrangement has a number of disadvantages.
First, the exhaust system and the oil tank compete for space in the already limited drive shaft housing. Secondly, it should be ensured that the exhaust gas heat does not readily dissipate to the oil pan to cause undue heating of the lubricant Furthermore, these types of systems generally require a long exhaust pipe that extends below the lower end of the oil pan and hence in a location where the exhaust pipe may be in proximity to the water level, under many running conditions.
This latter problem is particularly acute when engine speed is controlled by cylinder disabling, as is frequently done to permit smooth operation at low speeds. When cylinder disabling is encountered, negative pulses may exist in the exhaust pipe and these could draw water back upwardly toward the engine cylinder. Even if cylinder disabling is not employed, if the engine performance is to be improved by extending the intake period so that the intake valve is open before the exhaust valve closes, negative pressures may be experienced in the exhaust conduit. Thus, like two cycle engines there is a risk that the negative pressure may draw water into the engine through the exhaust system.
It is, therefore, a principal object of this invention to provide an improved, simplified and compact exhaust arrangement for a four-cycle outboard motor.
It is a further object of this invention to provide an exhaust system for a four-cycle outboard motor wherein the exhaust pipe is configured relative to the oil pan so as to provide silencing and also to ensure against water intrusion into the engine.
In addition to the main exhaust gas discharge problems afore-described, the utilization of an above the water exhaust gas discharge also is complicated when the exhaust system passes in part through the oil pan. With such arrangements, the idle exhaust must be delivered downwardly below the oil pan and then find the path back up to the above the water exhaust gas discharge. This can result in high back pressure and also the possibility of water intrusion.
It is, therefore, a still further object of this invention to provide the improved idle exhaust gas discharge arrangement for a four cycle outboard motor.
It is a yet further object of this invention to provide an improved idle exhaust gas discharge for a four cycle outboard motor wherein good silencing is accomplished and the cooling water need not be primarily employed to assist in the silencing.
When the exhaust pipe passes through or in proximity to the oil pan, then it is desirable to ensure against heat transfer, as discussed above. Also the exhaust gas impingement on the oil pan surface may cause corrosion. One way heat transfer and corrosion of the oil pan can be reduced or controlled is by discharging at least some of the cooling water from the engine also in the area around the exhaust pipe and between the exhaust pipe and the oil pan. This, however, further increases the risk of water becoming entrapped in the exhaust system or entering through the idle exhaust gas discharge.
It is, therefore, a still further object of this invention to provide an improved idle exhaust gas discharge arrangement for a four-cycle outboard motor wherein the exhaust gas can be discharged in proximity to the cooling water from the engine but wherein they are shielded from this cooling water or otherwise protected from water intrusion so that water cannot enter the engine through the idle exhaust gas discharge passage.